Calculating the number of pixels inside a freehand region.

I have a binary image, what I want to do is select a freehand region and then find out the total number of white and black pixels inside that region.

Answer 1

[Spandan Tiwari]

Since you are already using imfreehand, you can use the method named createMask available in the imfreehand object to rasterize the marked region into a binary image. Then you can just use logical AND to combine the binary image with your original image and sum over all the pixels in the image to get the desired count. I think the code would look something as follows. Say your image is called I.

figure, imshow(I)
f = imfreehand; 
% Then draw the region using imfreehand tool

bw = createMask(f);
outI = bw.*I;
numPixelsInsideRegion = sum(outI(:))

[Image Analyst]

Don't you need to cast bw to uint8 so that it's the same class as I before you multiply them?

[Spandan Tiwari]

Since the OP mentioned that it is a two-level image, I am assuming that the class of I is logical. If it' not, then it can be casted to logical by doing logical(I).

[Rukevwe Rim-Rukeh]

can you loop the freehand command, so you dont have to continously draw the area

[Image Analyst]

Sure. Just put it in a loop and OR it into the "master" binary image

finalMask = false(rows, columns)
for k = 1 : 10
    thisMask = (however you get it, like with drawfreehand());
    finalMask = finalMask | thisMask;
end

Answer 2

[Image Analyst]

This is exactly what my freehand drawing demo does. See attached.

[Salman]

Hi

Thanks Image Analyst, but i am afraid that what your demo does is a little different than what I want. I have a binary image, and i want to select a freehand region and compute the number of black and white pixels individually. What your demo dose is that it gives me the total number of pixels of the freehand region that i draw.

[Image Analyst]

Post your image and tell me what you're tracing around. If you have a pure black and pure white image rather than gray scale, you'll have to multiply the mask by your image before summing so that you'll get only the few pure black and pure white pixels in the freehand mask area rather than counting the total number of pixels in the area.

[Salman]

This is the image

and let's say that this is the free hand region that I select

As you can see that the image contains only two colors, i.e. black and white. Now what I want is, to find the total number of white pixels inside the freehand region that I draw and also the number of black pixels inside the freehand region.

I'll be grateful for any help.

P.S. I'm a newbie to image processing.

[Image Analyst]

See my demo to get the coordinates of the hand-drawn shape. Let's call them x, y, so first create a mask with poly2mask, then use nnz()

[imageRows, imageColumns = size(originalBinaryImage);
insideShape = poly2mask(x, y, imageRows, imageColumns);
newMask = originalBinaryImage & insideShape;
pixelCount = nnz(newMask)